The present invention relates to a variable displacement compressor that controls the pressure in crank chamber by supplying refrigerant in the discharge-pressure region of the compressor to the crank chamber and discharging refrigerant from the crank chamber to the suction-pressure region of the compressor, thereby controlling the displacement of the compressor.
In a variable displacement compressor having a crank chamber in which a swash plate is disposed so that its inclination angle is variable, the inclination angle of the swash plate decreases as the pressure in the crank chamber rises. This decrease of the inclination angle decreases the stroke length of a piston thereby to decrease the displacement of the compressor. On the other hand, the inclination angle of the swash plate increases as the pressure in the pressure control chamber falls. This increase of the inclination angle increases the stroke length of the piston thereby to increase the displacement of the compressor.
Since compressed refrigerant is supplied to the crank chamber in the variable displacement compressor, the operating efficiency of the variable displacement compressor deteriorates with an increase of the amount of refrigerant discharged from the crank chamber to the suction-pressure region. Therefore, the cross-sectional area of the bleed passage through which the refrigerant is discharged from the crank chamber to the suction-pressure region should be made as small as possible from the point of view of the operating efficiency of the variable displacement compressor.
When the variable displacement compressor is at a stop for a long time, the refrigerant in the crank chamber is liquefied and remains there. If the cross-sectional area of the bleed passage is fixed at a small value, the liquefied refrigerant in the crank chamber cannot be discharged to the suction-pressure region rapidly when the variable displacement compressor is started. The liquefied refrigerant in the crank chamber is vaporized during the start-up of the compressor, so that the pressure in the crank chamber is increased excessively. Therefore, it takes a long time before the displacement of the variable displacement compressor increases to a desired level after the compressor is started.
Japanese Patent Application Publication No. 2002-21721 discloses a displacement control unit for a variable displacement compressor for solving the problem mentioned above. The displacement control unit of the publication includes a first control valve for varying the cross-sectional area of a supply passage through which refrigerant is supplied from the discharge-pressure region to the crank chamber, and a second control valve for varying the cross-sectional area of a bleed passage through which refrigerant is discharged from the crank chamber to the suction-pressure region. The first control valve is an electromagnetically-operated valve that varies the valve opening by changing its electromagnetic force. When no electric current flows in the first control valve, its valve opening is maximized and the inclination angle of the swash plate is minimized. Thus, the compressor is operated at its minimum displacement. When an electric current flows in the first control valve, its valve opening is made smaller than the maximum opening and the inclination angle of the swash plate is made larger than the minimum, accordingly. Thus, the compressor is operated at an intermediate displacement where the displacement is not fixed at the minimum displacement.
The second control valve has a spool disposed in a spool chamber. The spool is a valve member for varying the cross-sectional area of the bleed passage and dividing the spool chamber into an internal space and a backpressure chamber. The backpressure chamber communicates with a pressure region located downstream of the first control valve, and the internal space communicates with the crank chamber via the bleed passage. The spool is urged toward the backpressure chamber by a spring. The spool is formed with a communication groove for providing a minimum cross-sectional area of the bleed passage. When the compressor is started, the first control valve is closed to move the spool of the second control valve in the direction that increases the cross-sectional area of the bleed passage. Thus, the liquefied refrigerant in the crank chamber is discharged to the suction-pressure region rapidly. Therefore, the time taken before the displacement of the compressor increases after the compressor is started is reduced.
When the first control valve is energized and placed in its open position, the second control valve is placed in its closed position where the spool is seated on its valve seat. Thus, discharging of the refrigerant from the crank chamber to the suction-pressure region is performed only via the communication groove. In this case, the compressor is operated at an intermediate displacement that is greater than the minimum displacement.
As the cross-sectional area of the communication groove is made smaller, the pressure in the internal space of the spool when the second control valve is in its closed position is made closer to the pressure in the crank chamber. When the opening of the first control valve is restricted, the pressure in the backpressure chamber is only slightly larger than the pressure in the internal space of the spool.
In order to move the second control valve to the closed position under the condition that the pressure in the backpressure chamber is slightly larger than the pressure in the internal space, the urging force of the spring needs to be reduced.
When the second control valve is moved from the closed position to the open position, the spool seated on the valve seat is moved away from the valve seat. The second control valve is formed so that the spool divides the spool chamber into the internal space and the backpressure chamber with a small clearance between the outer circumferential surface of the spool and the inner circumferential surface of the spool chamber. Therefore, if the ingress of any foreign matter into the clearance between the outer circumferential surface of the spool and the inner circumferential surface of the spool chamber may impede the operation of the spool. If the urging force of the spring is too small or no spring is present, the spool cannot move smoothly. That is, if the responsiveness of the second control valve is prevented by the foreign matter, the liquefied refrigerant in the crank chamber cannot be discharged to the suction-pressure region smoothly when the compressor is started.
The present invention is directed to a variable displacement compressor which prevents the responsiveness of its second control valve from deteriorating.